The present invention relates to polymeric materials and further to a process for the production of polymeric materials.
Rein et al., in PCT Application No. WO 97/11037, which is incorporated by reference for all purposes as if fully set forth herein, describe a process for producing a polyolefin composite material from an assembly of polyolefin fibers by treating the assembly with a solvent such as xylene, or with a solution of the polyolefin, to swell the surficial layers of the fibers, growing and crystallizing xe2x80x9cbrush layersxe2x80x9d on the fiber surfaces, and then heating and compressing the fibers. Preferably, the fibers are first placed under tension, the swelling of the surficial layers is effected at a temperature greater than that needed to melt the unloaded fibers but less than that needed to melt the loaded fibers, and the brush layers are grown and crystallized at a lower temperature.
Harpell et al., in European Patent Application 0 116 845, describe a process for transforming a network of high molecular weight polyethylene fibers into polyethylene articles, by the simultaneous application of a temperature between 100xc2x0 C. and 160xc2x0 C. and high pressure. The pressure is applied long enough to attain the desired degree of fiber coalescence, from simply causing adjacent fibers to adhere, to obtaining a film-like article which is substantially free of voids.
Ward et al., in U.S. Pat. No. 5,628,946, which is incorporated by reference for all purposes as if fully set forth herein, describe a process for producing a polymer sheet. An assembly of oriented polymer fibers is compressed at a contact pressure sufficient to hold the fibers in mutual intimate contact and then heated to a temperature sufficient to induce partial melting of the fibers. The melt fills the voids between the fibers. The assembly then is maintained at that temperature while being compressed at a still higher pressure to form the final product. In some cases, for example when the polymer fibers are made of highly oriented gel spun polyethylene, there is a tradeoff in the final product between high strength in the direction of fiber alignment, obtained by only limited partial melting, and high strength transverse to the direction of fiber alignment, obtained by more extensive partial melting.
Klocek et al., in U.S. Pat. Nos. 5,879,607 and 5,573.824 describe a method of making a protective coating material and the resultant high strength, high modulus continuous polymeric material for durable, impact resistant applications material. This method requires the use of overlapping layers of sinusoidal strands which results in relatively thick sheets.
According to the present invention there is provided a process for the production of a consolidated polymeric monolith from a thermoplastic polymer, the process including the steps of (a) forming an assembly of a thermoplastic polymer; (b) applying a pressure to the assembly sufficient to deform the polymer to substantially fill a majority of voids in the assembly, the assembly having a melting temperature at the deformation pressure; (c) heating the assembly to a temperature below the melting temperature but at which the assembly would at least partly melt at a transition pressure lower than the deformation pressure; and (d) subsequently reducing the applied pressure to the transition pressure while maintaining the assembly at the temperature for a time sufficient for the assembly to at least partly melt, thereby substantially filling a remainder of the voids.
It has been discovered that the process taught by Rein et al. in WO 97/11037 can be implemented with excellent results without preloading the fibers, provided that the assembly of solvent-treated or solution-treated fibers is compressed to a deformation pressure sufficiently high to fill most of the voids of the assembly by mechanically deforming the fibers. While under this deformation pressure, the assembly is heated to a temperature that is too low to melt the fibers while they are maintained at the deformation pressure, but that is high enough to at least partly melt the fibers at a lower pressure, referred to herein as the transition pressure. The assembly is maintained at this temperature, and the pressure is reduced to the transition pressure long enough to induce sufficient partial melting to substantially complete the filling of the voids but not long enough to impair the mechanical strength of the final product. Finally, the pressure on the assembly is increased to a consolidation pressure at least as great as the deformation pressure, in order to stop the melting, and the assembly is cooled to the ambient temperature at the consolidation pressure. If necessary, the cycling through transition pressure and consolidation pressure is repeated one or more times, at a frequency between 0.001 Hz and 0.5 Hz. The swelling of the fibers continues, albeit at slower rate, during the application of pressure and heat to the assembly of solvent-treated or solution-treated fibers. furthermore, it has been found that it is not necessary to add solvent to the fiber assembly, as taught by Rein et al. The residual solvent and/or lubricant left over on the polyolefin fibers after the manufacture of these fibers and/or weaving of the fiberbased cloths is sufficient to initiate the desired fiber consolidation. This residual solvent and/or lubricant generally constitutes less than 2% of the fiber assembly by weight.
Unlike the process taught by Ward et al., the present invention is applicable to both oriented and unoriented fiber assemblies. Furthermore, the present invention is also suited to use with polymer in non-fiber forms, including, but not limited to, powder, beads, tape chips and discs.
The present invention allows more precise control of conditions within the assembly than the process taught by Ward et al. The heat applied to the assembly is necessarily applied from outside of the assembly. As a result, the temperature field inside the assembly is inhomogeneous, at least initially, and different portions of the assembly undergo different degrees of partial melting. According to the present invention, the assembly is first subject to pressure and heat without melting, and then the pressure is released to induce the partial melting. Because the pressure change is propagated throughout the assembly essentially instantaneously, while the temperature field in the assembly is homogeneous, the partial melting of the assembly is induced uniformly, without any limitation on the thickness of the assembly.
Unlike the process taught by Ward et. al., the present invention makes use of straight fibers arranged in overlapping layers which may be at any angle to one another. This process allows production of plastic sheets with a minimum thickness approaching the fiber filament diameter, a significant improvement over xe2x80x9coverlapped sinusoidal strandsxe2x80x9d based products produced by following the teachings of Klocek et al. This improvement is especially important in infrared sensing applications and high frequency dielectric material applications where both strength and thinness are required.
The scope of the present invention also includes articles of manufacture made by the process of the present invention.
According to preferred embodiments of the present invention, these articles of manufacture, in the form of polymeric monoliths are characterized by a density greater than that available using methods taught by the prior art. This density is an indication of strength so that the present invention represents an important improvement over the prior art.
According to preferred embodiments of the present invention, the polymeric monoliths are further characterized by super-oriented needlelike voids.
According to preferred embodiments of the present invention, the polymeric monoliths are further characterized by a high level of matrix orientation.
According to further features of preferred embodiments, the aforesaid articles of manufacture exhibit more than 70% transmission of most wavelengths between 2 and 12 micrometers, a quality which is critical for many optical sensor applications.
According to additional further features of the present invention, the aforesaid articles of manufacture are characterized by a low thermal expansion coefficient.
According to still further additional further features of the present invention, the aforesaid articles of manufacture are characterized by predetermined thermal expansion coefficients for each of the three axes X, Y, and Z.
According to still further features of the present invention, the aforesaid articles of manufacture bond not less strongly to one another when welded or dissolved together than when bonded with a strong polar glue, such as epoxy.
According to still further features of the present invention, the aforesaid articles of manufacture bond not less strongly to other surfaces, for example a metal, when welded or dissolved thereon than when bonded with a strong polar glue, such as epoxy.
The range of articles of manufacture included in the present invention, and produced by the process of the present invention, have significant advantages over those produced according to prior art teachings with respect to a wide range of applications, including, but not limited to, ballistic protection, radomes for communications antennae, electronic circuit boards, medical X-ray imaging applications, orthopedic implants, and membranes for industrial waste processing.